Frye FP35 Manual

A SERVICE OF

next previous

40 FONIX FP35 Hearing Aid Analyzer

• FLATwithDSLfittingruleandpure-tonesignalabove85dBSPL

• A-LTASSwithDSLfittingruleforadults(allothersignaltypes)

• C-LTASSwithDSLfittingruleforchildren(allothersignaltypes)

2.5.2.2 Intermodulation Distortion

IM distortion occurs when more than one frequency is present in the source sig-

nal and those frequencies combine to create new frequencies not actually pres-

ent in the source. It is visible as jagged peaks in the frequency response of the

hearing aid when a composite signal source is used.

The IM distortion test was developed as a more precise measurement of the

amount of IM distortion created by the hearing aid than can be obtained by just

eyeing the jagged peaks of the response curve to a composite source. It works

by presenting two different frequencies to the aid at the same time and measur-

ing the response of the aid to those frequencies. IM distortion occurs when the

aid produces components at frequencies other than the two input frequencies.

These additional components usually occur at frequencies that are mathemati-

cally related to the input frequencies (multiples of the sum and difference of the

original frequencies and their harmonics).

The two input frequencies are known as the “primary” and the “secondary” fre-

quencies. The primary frequency is controlled either by the user with the [<, >]

keys, or by the analyzer when it is doing a sweep test. The secondary frequency

is always at a fixed distance from the primary frequency. You can select the

fixed distance between the primary and the secondary frequency in a menu of

the measurement screen.

Types of IM distortion tests

You can perform a static IM distortion test or an IM distortion sweep. In the

static IM distortion test, the FP35 analyzer produces a signal consisting of the

primary and secondary frequencies, and shows the resulting response as a com-

posite type display. The measurement is updated in real-time until you turn it

off. You can change the primary frequency by using the [<, >] keys. The sec-

ondary frequency will also adjust as you change the primary. In this manner,

you can manually step through the different frequencies and see when the aid

produces IM distortion.

In the IM distortion sweep test, the FP35 analyzer produces a sweep of the two

frequency combination. This is similar to a normal pure-tone frequency sweep,

but two frequencies are used at a time instead of one at a time. The FP35 ana-

lyzer measures the IM distortion by looking at frequencies that have a math-

ematical relationship to the primary frequencies. Any amplitudes at those fre-

quencies is considered IM distortion. The frequencies chosen are selected in the

DISTORTION menu selection. See the next section for details. The results are

displayed as a double wide line with a percentage scaling found on the right

side of the graph.